The present invention relates generally to implantable medical devices and methods, and more particularly to a system and associated method for use with an implantable stimulation device, such as a pacemaker or a cardioverter-defibrillator device (ICD) to automatically determine the onsets and ends of cardiac events such as R-waves and T-waves, and far-field signals such as far-field R-waves and far-field T-waves.
Conventional pacemakers and ICDs require manual programming of numerous programmable parameters including but not limited to: ventricular sensitivity, atrial sensitivity, post-ventricular atrial refractory period (PVARP), post-ventricular atrial blanking period (PVAB), ventricular refractory period (VREF), and other parameters such as ventricular output, atrial output, choice of pacing mode, upper rate limit, base rate, sleep rate, sensor slope, sensor threshold, and so forth. The programming of these parameters can be inaccurate and time consuming, and requires highly-skilled medical expertise to accomplish. Attempts to automate the programming of these parameters have not been completely successful, in part because of the inaccuracy of determining the onsets and ends of cardiac events such as P-waves, R-waves and T-waves, and far-field signals such as far-field R-waves and far-field T-waves.
For example, when sensing in the atrium, PVAB is a key parameter in the correct performance of automatic mode switching, in that an incorrectly short PVAB would result in atrial over-sensing and inappropriate mode switching, and an overly long PVAB would prevent the correct detection of atrial fibrillation and would result in inappropriate ventricular pacing during atrial fibrillation, with serious hemodynamic consequences. In addition, under-sensing of premature atrial contractions (PACs) and premature ventricular contractions (PVCs) can result in arrhythmia induction.
Therefore, there is a great and still unsatisfied need for a system and method which not only discriminate between sensed cardiac events such as P-waves, R-waves and T-waves, as described in U.S. Pat. No. 5,782,888 to Sun et al., and far-field signals such as far-field R-waves and far-field T-waves, but which will also automatically and accurately determine the onsets and ends of these events and signals.
The problem of automatically and accurately sensing P-waves, R-waves, and T-waves is even more pronounced when using an xe2x80x9cA-V combipolarxe2x80x9d electrode configuration, that is, an electrode configuration in which the stimulation device senses cardiac signals between an atrial tip electrode and a ventricular tip electrode, and stimulates each chamber in a unipolar fashion from the respective electrode to the housing (i.e., typically referred to as the case electrode). For a more complete description of combipolar systems, see U.S. Pat. No. 5,522,855 (Hognelid), which reference is incorporated herein by reference. When such electrodes are implanted, various electrode sensing configurations are possible, e.g., atrial unipolar (A tip-case); ventricular unipolar (V tip-case); atrial-ventricular combipolar (A tip-V tip); ventricular unipolar ring (V ring-to-case) or atrial unipolar ring (A ring-to-case).
More specifically, regardless of the cardiac event being sensed, and regardless of the electrode configuration being used, there is a need for an implantable device that is able to readily and reliably distinguish between P-waves, R-waves and T-waves. This is because the implantable device, if it is to perform its intended function, must know when an atrial depolarization (P-wave) occurs, and when a ventricular depolarization (R-wave) occurs, and it must not falsely sense a T-wave or noise as a P-wave or R-wave.
For example, it is of critical importance that the implantable device be capable of recognizing the occurrence of certain atrial arrhythmias based on the sensed atrial rate, and in determining such rate it is critically important that neither far field R-waves nor far field T-waves be falsely sensed as a P-wave. Such may be particularly noticeable when an A-V combipolar electrode configuration is being used because, in such configuration, P-waves, R-waves, and T-waves may be sensed as being of the same order of magnitude.
While it is well known that various blanking schemes may be used to block or blank out unwanted T-waves and retrograde P-waves by using different blanking intervals (i.e., PVARP, automatic PVARP extension, PVAB, etc.), and thereby prevent these T-waves or retrograde P-waves from being falsely sensed as P-waves, such blanking schemes (based solely on timing considerations) have proven less than satisfactory because legitimate (antegrade) P-waves and PVCs that need to be sensed, may and do occur during these blanking intervals.
Differentiation schemes based on the morphology of the sensed waveform have also been used. These schemes are premised on the fact that P-waves, R-waves and T-waves have inherently different shapes. Thus, in theory, all one needs to do is to examine the morphology of the sensed waveform. Unfortunately, morphology-based schemes require that the entire waveform be captured and analyzed, a process that not only requires waiting until the entire waveform has occurred, but also may require significant on-chip processing capability and processing time.
Thus, it is seen that there is a need in the implantable cardiac stimulator art to automatically and accurately detect, discriminate, and determine the onsets and ends of cardiac events such as P-waves, R-waves and T-waves, and far-field signals such as far-field R-waves and far-field T-waves, without relying solely on blanking considerations or morphology. This need becomes particularly acute when sensing between intra-chamber electrodes, e.g., when sensing using an A-V combipolar electrode configuration.
Thus, it is seen that there is a need for an implantable cardiac stimulator that automatically and accurately detects, discriminates, and determines the onsets and ends of cardiac events such as R-waves and T-waves, and far-field signals such as far-field R-waves and far-field T-waves, without relying solely on blanking considerations or morphology.
The present invention addresses these problems by providing a method for automatically determining the onset and termination of cardiac events, namely the R-wave, the T-wave and the far-field signals sensed in the atria associated with the R-wave, commonly referred to as the far-field R-wave (FFR) and T-wave, commonly referred to as the far-field T-wave (FFT). Furthermore, having determined the temporal location and duration of these events as well as their peak amplitudes, the present invention provides a method for automatically setting various pacemaker parameters, specifically PVAB, PVARP, VREF, atrial sensitivity, and ventricular sensitivity.
First, an algorithm is executed based on command logic stored in the control system of the stimulation device which determines the onset and termination of the cardiac events. The onset is defined as the time of the first sampled point of the cardiac signal whose magnitude exceeds a pre-defined threshold for the particular event. Once the onset of an event is positively determined, the cardiac signal is sampled at given intervals. The change in magnitude of these sampled points is determined.
The termination of the event is identified through an algorithm that compares the difference in magnitude of these sampled points. During an event, whether it be an R-wave, T-wave, far-field R-wave (FFR) or far-field T-wave (FFT), the difference in amplitude between one sampled point and another sampled point a given time interval later will be large as long as the action potential within the cardiac muscle tissue is generated. However, toward the end of these events, the difference in amplitude between sampled points will diminish. The end of a particular cardiac event, therefore, can be recognized by comparing these changes in amplitude, and defining the termination as the sampled point after which the change in amplitude no longer exceeds a given value.